1. Field of the Invention
The present invention relates to a photoelectric conversion device using a semiconductor substrate, and more particularly to a solar battery. The structure of the solar battery is applicable to the solar batteries of various types using a bulk semiconductor such as a known mono-crystal wafer or polycrystal wafer.
Also, the present invention relates to a photoelectric conversion device using a thin film semiconductor formed on an insulating or conductive substrate for a photoelectric conversion layer, and the solar battery using the thin film semiconductor is applicable to the solar batteries of various types.
The present invention is applicable to a junction type solar battery based on a p-n junction, a non-junction type solar battery having a Schottky barrier, a MIS structure or the like, a multi-layer junction type solar battery, a hetero junction type solar battery and so on.
2. Description of the Related Art
The solar batteries can be manufactured using a variety of semiconductor material or organic compound material, however, from the industrial viewpoint, silicon which is semiconductor is mainly used for the solar batteries. The solar batteries using silicon can be roughly classified into a bulk type solar battery using a wafer of mono-crystal silicon, polycrystal silicon or the like and a thin film solar battery having a silicon film formed on a substrate.
Also, the reduction of the manufacture costs has been required for the spread of the solar batteries, and in particular, the thin film solar battery has been expected to provide the effects of the reduced costs because the raw material used for the thin film solar battery is reduced in comparison with the bulk type solar battery.
At present, in the field of the thin film solar battery, an amorphous silicon solar battery has been put to practical use. However, because the amorphous silicon solar battery is lower in conversion efficiency than the solar batteries using mono-crystal silicon or polycrystal silicon, and also suffers from problems such as light deterioration, its use is limited. For that reason, as another means, the thin film solar battery using a crystalline silicon film has also been developed.
As mentioned above, although the high conversion efficiency and the reduction of the manufacture costs are required at the same time for the solar battery, both are substantially contrary to each other. For example, in the case of pursuing the conversion efficiency, it can be relatively readily achieved by using the mono-crystal wafer of a high grade (defects and the like are remarkably reduced) although the manufacture costs are increased as much.
On the contrary, under existing circumstances, even though the costs can be reduced by using the mono-crystal wafer of a low grade (so-called solar battery grade, etc.), the conversion efficiency is slightly lowered unavoidably. In particular, the polycrystal wafer, the thin film solar battery and so on have been developed for primarily reducing the costs, so that the conversion efficiency is subordinate to the reduction of the costs.
Also, in view of the low costs, attention has been paid particularly to the thin film solar battery using the crystalline silicon thin film. However, since the crystalline silicon thin film, for example, the mono-crystal silicon thin film is small in absorption coefficient, the thin film solar battery suffers from such a problem that it does not sufficiently function as the photoelectric conversion device without thickening the film. Since a grain boundary always exists in the polycrystal silicon thin film generally obtained, the grain boundary forms an electronic state having an energy corresponding to a forbidden band, to thereby shorten the lifetime of carriers. In other words, when the film is thickened, the carriers are recombined before they reach an electrode, thereby making it difficult to ensure a sufficient photoelectric conversion efficiency.